1. Field of the Invention
This invention relates to a method for the transmission of a cyclic data, which performs operations to bring the data in a shared cyclic memory up to date in a distribution control transmission system wherein a synchronous transmission for a cyclic data and an asynchronous transmission for an event are both included.
2. Description of the Prior Art
FIG. 1 is a block diagram showing one embodiment of one of stations which constitute a control data way for realizing a protocol of FDDI (Fiber Distributed Data Interface) which has been introduced in, for example, the technical journal entitled "Telecommunication" Vol. 21, No. 5 (issued May 1987).
Referring to FIG. 1, designated at numeral 1 is a circuit, namely, a communication channel while numeral 2 indicates a circuit connection unit. Designated at numeral 3 is a media access unit connected to the circuit connection unit 2 and equipped with a token synchronous timer 3a and a token holding timer 3b.
Numeral 4 indicates a buffer memory which is operative to transmit data to the media access unit 3 and receive same therefrom. The buffer memory 4 is provided with a transmission buffer memory 4a and a receiving buffer memory 4b. Designated at numeral 5 is a DMA transfer controller for controlling access to the buffer memory 4. Numeral 6 indicates a cyclic memory for storing synchronous data which require a real-time processing (namely, the performance of calculations in real time). Designated at numeral 7 is an event transmission controller for controlling asynchronous data which do not require real-time processing.
Numeral 8 indicates an external controller for controlling the cyclic memory 6 and the event transmission controller 7. Incidentally, one of stations 9 is constructed of the circuit connection unit 2, the media access unit 3, the buffer memory 4, the DMA transfer controller 5, the cyclic memory 6 and the event transmission controller 7 as shown in FIG. 1.
FIG. 2(a) and (b) show an allocation of job data assigned to frames, in which FIG. 2(a) is a diagram showing the allocation of a token frame 10. In the token frame 10, there are provided a start delimiter 10-SD, a frame control address 10-FC, a destination address 10-DA, a transmission source address 10-SA, a frame check sequence 10-FCS and an end delimiter 10-ED.
In addition, FIG. 2(b) is a diagram showing the allocation of a data frame 11. In the data frame 11, there are provided a start delimiter 11-SD, a frame control address 11-FC, a destination address 11-DA, a transmission source address 11-SA, an information area 11-INFO, a frame check sequence 11-FCS, an end delimiter 11-ED and a frame status 11-FS.
FIG. 3 is an overall schematic view of a control data way which has been constructed by using the above-described stations. In the drawing, a plurality of stations 9a, 9b and 9c are connected to a looped transmission path 12. As also shown in FIG. 1, each of the stations 9a, 9b and 9c is provided with the media access unit 3, the cyclic memory 6 and the event transmission controller 7. External controllers 8a, 8b and 8c are connected to their corresponding stations 9a, 9b and 9c.
Next, the operation of the above exemplary frames will be described. When one of the stations 9 connected to the looped transmission path 12 acquires an access to the token frame 10 of the allocation shown in FIG. 2(a), the station takes priority of the transmission loop, namely, becomes ready to transmit information of the data frame 11 of the allocation illustrated in FIG. 2(b). At this time, values obtained by the token rotation timer 3a are duplicated to the token holding timer 3b and the token rotation timer 3a is established (namely, reset) to a predetermined value, whereby the counting of time is resumed. Here, the token rotation timer 3a has already started counting the time from the time of capture of the preceding token. This has been done by subtracting from a preset value the count of time from the capture of the preceding token to the capture of the present token, in other words, by performing a downcount. It is therefore possible to determine a data transmission enable time for asynchronous data by the values of the token rotation timer 3a at the time of the capture of the token. Values thus obtained are set in the token holding timer 3b. Incidentally, a preset value to be set in the token rotation timer is a token rotation target time (TRTT) which corresponds to a target value of a token rotation time.
At this time, transmission cyclic data from one of the stations 9 are queued in the transmission buffer memory 4a and are ready for sending as synchronous data. The transmission cyclic data are however transmitted to the others of the stations 9 when said priority for the transmission is taken.
As the timing at which the transmission cyclic data are transferred from the cyclic memory 6 to the transmission buffer memory 4a, the time of token capture, token issue or the like is used by way of example because it is uncertain how soon the next token will be inputted.
When the transmission of the synchronous data is terminated completely, the transmission of the asynchronous data stored in the transmission buffer memory 4a is performed until the token holding timer 3b is rendered expired. Here, the term expired means that the token holding timer 3b has counted downwards and indicates "0".
The token holding timer 3b stops counting the time during the transmission of the synchronous data and performs counting the time during the transmission of the asynchronous data frame. When the token rotation timer 3a has already been rendered to an expired state upon the token frame capture, the transmission of the asynchronous data is not performed.
According to this method, the time required to transmit the synchronous data and asynchronous data, namely, the token rotation time will not be in excess of a value obtained by resetting the token rotation timer 3a, namely, twice the token rotation target time.
Incidentally, this transmission method can always maintain the same quantity of synchronizing data in each station providing that the quantity of synchronizing data transmitted by the acquisition of a token, in other words, the quantity of transmission data of the station in the cyclic memory 6 itself is set so as to become equal each time. Further, the token circulating time will not be in excess of the token circulating target time which is preset in the token circulation timer 3a. A certain constant response is assured to occur to each of cyclic data, so that some improvements have been brought about.
The renewal of cyclic data has conventionally been performed by the above-described method. However, as the timing at which the cyclic data are transferred from the cyclic memory 6 to the transmission buffer memory 4a, the time of token capture or the time of token issue has been employed. Such a method has therefore been accompanied by the problem that it is necessary to take time of twice (or one time) the token rotation target time at the maximum for causing the cyclic data to be transmitted to a looped transmission path after the cyclic data has been transferred from the cyclic memory to the transmission buffer memory, so that a response relative to the cyclic data is reduced.